Cosmic Expansion Puzzle
26 JUNE 2019 • SKY & TELESCOPE
mysterious dark energy, the true nature of which is one of the
biggest mysteries in science (S&T: May 2018, p. 14).Cosmology Crisis
No one realized it at the time, but the discovery of the accel-
erated expansion of the universe germinated the current cri-
sis in cosmology that was the topic of the Berlin symposium.
Not because the concept of dark energy is somehow defi cient,
but because it works too well.
Over the past 20 years, astronomers have come to realize
that we live in a weird universe, dominated by dark energy
(denoted by the Greek letter lambda, Λ) and (cold) dark
matter (CDM), which is every bit as mysterious (S&T: Aug.
2017, p. 28). But although we don’t know the true nature
of these enigmatic components, the ΛCDM model of the
universe successfully accounts for all kinds of cosmological
observations, including the large-scale clustering properties
of galaxies.
In particular, the ΛCDM model appears to be the only
one that is compatible with the observed properties of the
cosmic microwave background. The statistical distribution
of the “hot” and “cold” spots in the CMB (in fact, tempera-
ture differences of less than a ten-thousandth of a degree),
as observed in fi ne detail by the European Planck spacecraft,
can only be understood if the universe is largely made up of
dark energy and dark matter (S&T: July 2015, p. 28). From
the 2018 fi nal Planck data release, cosmologists conclude
that 68.4% of the matter-energy density of our universe is
accounted for by dark energy; 26.5% by dark matter (probably
some as-yet-undiscovered type of elementary particle), and
no more than 4.9% by ordinary matter, consisting of atoms
and molecules. (These fractions don’t quite add up to 100%
because of rounding and the still-uncertain neutrino mass,
among other reasons.)As Antony Lewis (University of Sussex, UK) told the Berlin
audience, these cosmological parameters have now been
derived so precisely that it’s easy to deduce what the current
value of the Hubble constant should be: 67.4 km/s/Mpc, with
an error margin of less than 1%. (This deduction takes into
account the fact that the cosmic expansion rate fi rst slowed
down because of the universe’s self-gravity but is now speed-
ing up again, because dark energy started to prevail some
6 billion years ago.) And there appears to be very little wiggle
room, says Colless. “It’s hard to get rid of [this result] without
running into all kinds of other problems.”
The same value is arrived at by completely independent
results from the Dark Energy Survey (DES). Carried out atthe Cerro Tololo Inter-American Observatory in Chile, DES
looked at the clustering properties of galaxies and at weak
lensing — the tiny “shape-shifting” of remote galaxies due to
the light-bending gravity of foreground galaxies and clusters
(S&T: Sept. 2016, p. 34). These results have an error margin of
about 2%, says DES theoretical cosmologist Dragan Huterer
(University of Michigan).
But the lack of wiggle room in these results is causing
a problem of its own: They don’t jibe with updated “local”
measurements of H 0 from Cepheids and supernovae. The
2001 result from Freedman’s Hubble Key Project had a large
enough range of uncertainty that, at fi rst, there didn’t seem
to be cause for concern. But over the past years, a team led
by Riess has arrived at a much more precise calibration ofTemperaturefluctuations(mic
rokelvi(^2) n
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Angular separation
Step 1: Astronomers measure the patchiness of
the cosmic microwave background. (Data from
Planck shown.)
How Astronomers Calculate H 0 from the CMB
Step 2:The map gives them the power spectrum,
which plots how large the temperature diff erences
are between two locations on the sky, depending on
how far apart those locations are.
No one realized it at the time, but the discovery
of the accelerated expansion of the universe
germinated the current crisis.
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